Apr 20, 2010 (CIDRAP News)  In the biggest and most detailed look yet at pandemic flu infections in pregnant women, researchers from the US Centers for Disease Control and Prevention (CDC) reported today that early antiviral treatment was linked to fewer intensive care unit (ICU) admissions and that severe illnesses and deaths are more likely to occur during the third trimester.

The CDC researchers, along with a Pandemic H1N1 Influenza in Pregnancy Working Group made up of health officials from several states, based their findings on reports of pregnant women who were sick with pandemic H1N1 infections through August plus more recent reports of women who were admitted to ICUs. The findings appear in the Apr 21 issue of the Journal of the American Medical Association (JAMA).

Though several reports of H1N1 infections in pregnant women have been reported in the medical literature, the new JAMA report is the first nationwide US summary to be published since early in the pandemic when the CDC reported on a series of 34 pregnant women, the authors noted.

In the early months of the pandemic, federal officials warned that pregnant women could be at higher risk for complications, as they were during previous pandemics. Pregnancy-related changes in immune, cardiac, and respiratory systems are thought to contribute to the greater risk.

As pandemic infections mounted last year, it became clear that the risk was real. Reports suggest that although pregnant women make up only about 1% of the US population, they accounted for 5% of all reported pandemic H1N1 deaths. Federal officials placed pregnant women at highest priority for the vaccine and have repeatedly urged clinicians to begin antiviral treatment early in pregnant patients, at the first sign of symptom onset.

Today's report pinpoints trends from reports of 788 flu cases in pregnant women that the CDC received through Aug 21 and explores reports of an additional 165 women who were admitted to ICUs from Aug 21 through Dec 31. Last August the CDC asked state health departments, along with some major metropolitan areas, to provide additional information such as gestational age, illness onset, and underlying conditions using a standardized form for all confirmed and probable pandemic infections in pregnant women.

In the middle of October the CDC launched its Pregnancy Flu Line and asked all state and metropolitan health departments to submit reports for severely ill pregnant womenthose who were admitted to the ICU or died.

Researchers found that pregnant women who began antiviral treatment more than 4 days after symptoms appeared were six times more likely to be admitted to an ICU than those who were treated within 2 days of symptom onset (56.9% versus 9.4%). Of the 588 women for whom treatment information was provided, 509 received treatmentabout 84% received oseltamivir (Tamiflu).

Women in their second and third trimesters were almost equally represented among the cases reported (42% and 46%), but of the 30 women with flu who died, 60% were in their third trimester, the group reported. Though severe illnesses occurred in all three trimesters, the proportion during the third trimester was higher.

Among other patterns to emerge from the data, Hispanic women were the most commonly reported racial group, though white women were at the top of the fatality list.

The CDC received reports of underlying conditions for 432 of the cases. The most common were asthma, obesity, and presentational or gestational diabetes. Among pregnant women who died, asthma was the most frequently reported underlying condition.

The group wrote that their findings confirm public health recommendations for early antiviral treatment and flu vaccination for pregnant women. They urged that pregnant women with flu symptoms receive empirical antiviral treatment without waiting for the results of diagnostic tests. They also said the data suggest an antiviral benefit, even when treatment is started as many as 4 days after symptom onset.

Findings support a continued push for pandemic flu vaccination in pregnant women, the group wrote. An earlier CDC study suggested that pandemic vaccine uptake was about 38% in pregnant women, which is higher than for the seasonal flu vaccine but not as high as health officials had hoped.

Kevin Ault, MD, associate professor of gynecology and obstetrics at Emory University School of Medicine in Atlanta, told CIDRAP News that antiviral therapy looks very impressive in the study. "It's clear that this is going to tip the balance toward treatment," he said.

Though the public health officials have advised early antiviral treatment for several months now, the study adds backbone to the recommendations, he said.

The medical literature has some data about uptake of the pandemic vaccine in pregnant women, but not much is known about use of antivirals, Ault said. He added that the American College of Obstetrics and Gynecology came out early with e-mail pitches about antiviral use in this group to physicians, which he says has helped build awareness in the medical community on the importance of early antiviral treatment for pregnant women.